Low & Flat

Beach One at 6:48 pm 1009 hPa. A low low tide, a wide flat beach and exposed sandbars.

Testing the sand drift

On the summer by the sea walk we tried a simple experiment. By quickly digging a trench in front of a wave, then we watched carefully as the backwash from the wave refilled the trench.

Thanks to steve for the video
If you watch, you will see the first and main backwash is from the top right (south eastern side), but then material also washes in from the top left (north western side). So the waves are not washing back down the beach in a uniform direction and maybe even our trench is influencing the backwash a little. Still we repeated the test three times with much the same result. So we must assume an inconclusive result that the sand is mostly washing to the north west (ie coming from the south east) which is consistent with the form of the sand bars, which joins to the beach at the southern end in this location.

A good day for a walk

I hope those that braved the walk out around Point Symthe area had a great day. One of the highlights was seeing a pair of hooded plovers possibly at a new nesting site and some oyster catchers resting on the beach.

The experiments with trying to directly observe sand drift, where not totally conclusive (steve may have photos) but overall seemed to show more movement to the north west than south east. This corresponds with the direction obtained by reading the sand bars, which where attached to the beach at the south eastern end.

Understanding Longshore Drift

When Waves arrive at the beach at an angle to the shore line, sand will be pushed up the beach in that direction however in the backwash the sand will usually moved directly down the beach, perpendicular to the shore line. So the sand follows a zigzag pattern. The result is a net movement of sand along the beach. An enormous amount of sand can moved in this manner.

By looking at new sand bars you can determine the longshore drift. If the sand bar is attached to the beach at its northern end the longshore drift is to the south. Conversely if the sandbar is attached to the beach at its southern end the longshore drift is to the north.

Longshore drift can fluctuate from day to day, and may vary locally as the beach tries to readjust its shape to be in better equilibrium with the waves and currents.